Yes, 4 matches.
Multiple reasons. First, for set minimum amount of matches... because some mangers lately I've seen splitting OOPs because "they feel like its not enough matches"
Okay, so then you get the Day 2 OOP, and they pick 2 matches that belong to day 1's matches and the rest are day 2. But how do they decide which matches are part of day 1? If they're obvious matches, then it really doesn't matter that they were added to the OOP at all really. I mean, does it REALLY help if there are 4 matches on Day 1, and you decide to add Federer vs Bozoljac and Djokovic vs Peliwo from day 2? You're going to get the same differences as you had the first day most likely. No one would pick Bozoljac (unless they've lost their minds
) So what good did it do
Anyway, that brings me to why I feel 4 is a good minimum. The SF is a vastly more important round and it is decided by 4 matches every single tournament
If we feel comfortable deciding a SF match on 4 matches, we should feel comfortable deciding a first round match by 4 matches.
There are few cases where there are only 3 singles matches on day 1, and in these cases I feel it is acceptable to add a match from day 2, but I think it should be avoided as much as possible, and why I feel 4 is an adequate amount.